Beyond Viral Neutralization

Conformational dynamics of the HIV-1 envelope glycoprotein from CRF01_AE is associated with susceptibility to antibody-dependent cellular cytotoxicity

The HIV-1 envelope glycoprotein (Env) is expressed at the surface of infected cells and as such can be targeted by non-neutralizing antibodies (nnAbs) that mediate antibody-dependent cellular cytotoxicity (ADCC). Previous single-molecule Förster resonance energy transfer (smFRET) studies demonstrated that Env from clinical isolates predominantly adopt a "closed" conformation (State 1), which is resistant to nnAbs. After interacting with the cellular receptor CD4, the conformational equilibrium of Env shifts toward States 2 and 3, exposing the coreceptor binding site (CoRBS) and permitting binding of antibodies targeting this site. We showed that the binding of anti-CoRBS Abs enables the engagement of other nnAbs that target the cluster A epitopes on Env. Anti-cluster A nnAbs stabilize an asymmetric Env conformation, State 2A, and have potent ADCC activity. CRF01_AE strains were suggested to be intrinsically susceptible to ADCC mediated by nnAbs. This may be due to the presence of a histidine at position 375, known to shift Env towards more "open" conformations. In this work, through adaptation of an established smFRET imaging approach, we report that the conformational dynamics of native, unliganded HIV-1CRF01_AE Env indicates frequent sampling of the State 2A conformation. This is in striking contrast with Envs from clades A and B, for example HIV-1JR-FL, which do not transition to State 2A in the absence of ligands. These findings inform on the conformational dynamics of HIV-1CRF01_AE Env, which are relevant for structure-based design of both synthetic inhibitors of receptor binding, and enhancers of ADCC as therapeutic alternatives.

Crimean-Congo hemorrhagic fever survivors elicit protective non-neutralizing antibodies that target 11 overlapping regions on glycoprotein GP38

Crimean-Congo hemorrhagic fever virus can cause lethal disease in humans yet there are no approved medical countermeasures. Viral glycoprotein GP38, exclusive to Nairoviridae, is a target of protective antibodies and is a key antigen in preclinical vaccine candidates. Here, we isolate 188 GP38-specific antibodies from human survivors of infection. Competition experiments show that these antibodies bind across 5 distinct antigenic sites, encompassing 11 overlapping regions. Additionally, we show structures of GP38 bound with 9 of these antibodies targeting different antigenic sites. Although these GP38-specific antibodies are non-neutralizing, several display protective efficacy equal to or better than murine antibody 13G8 in two highly stringent rodent models of infection. Together, these data expand our understanding regarding this important viral protein and may inform the development of broadly effective CCHFV antibody therapeutics.

Neutralizing the threat: harnessing broadly neutralizing antibodies against HIV-1 for treatment and prevention

Broadly neutralizing antibodies (bnAbs) targeting the human immunodeficiency virus-1 (HIV-1) have played a crucial role in elucidating and characterizing neutralization-sensitive sites on the HIV-1 envelope spike and in informing vaccine development. Continual advancements in identifying more potent bnAbs, along with their capacity to trigger antibody-mediated effector functions, coupled with modifications to extend their half-life, position them as promising candidates for both HIV-1 treatment and prevention. While current pharmacological interventions have made significant progress in managing HIV-1 infection and enhancing quality of life, no definitive cure or vaccines have been developed thus far. Standard treatments involve daily oral anti-retroviral therapy, which, despite its efficacy, can lead to notable long-term side effects. Recent clinical trial data have demonstrated encouraging therapeutic and preventive potential for bnAb therapies in both HIV-1-infected individuals and those without the infection. This review provides an overview of the advancements in HIV-1-specific bnAbs and discusses the insights gathered from recent clinical trials regarding their application in treating and preventing HIV-1 infection.

The molecular immune modulator adenosine deaminase-1 enhances HIV specific humoral and cellular responses to a native-like HIV envelope trimer DNA vaccine

There is currently no prophylactic vaccine available for human immunodeficiency virus (HIV). Research efforts have resulted in improved immunogens that mimic the native envelope (Env) glycoprotein structure. Recently, a novel triple tandem trimer (TTT) platform has been used to generate a plasmid encoding Env immunogen (pBG505-TTT) that expresses only as trimers, making it more suitable for nucleic acid vaccines. We have previously demonstrated that adenosine deaminase-1 (ADA-1) is critical to the T follicular helper (TFH) function and improves vaccine immune responses in vivo. In this study, we demonstrate that co-delivery of plasmid-encoded adenosine deaminase 1 (pADA) with pBG505-TTT enhances the magnitude, durability, isotype switching and functionality of HIV-specific antibodies in a dose-sparing manner. Co-delivery of the molecular immune modulator ADA-1 also enhances HIV-specific T cell polyfunctionality, activation, and degranulation as well as memory B cell responses. These data demonstrate that pADA enhances HIV-specific cellular and humoral immunity, making ADA-1 a promising immune modulator for HIV-targeting vaccines.

Crimean-Congo Hemorrhagic Fever Survivors Elicit Protective Non-Neutralizing Antibodies that Target 11 Overlapping Regions on Viral Glycoprotein GP38

Crimean-Congo hemorrhagic fever virus can cause lethal disease in humans yet there are no approved medical countermeasures. Viral glycoprotein GP38, unique to Nairoviridae, is a target of protective antibodies, but extensive mapping of the human antibody response to GP38 has not been previously performed. Here, we isolated 188 GP38-specific antibodies from human survivors of infection. Competition experiments showed that these antibodies bind across five distinct antigenic sites, encompassing eleven overlapping regions. Additionally, we reveal structures of GP38 bound with nine of these antibodies targeting different antigenic sites. Although GP38-specific antibodies were non-neutralizing, several antibodies were found to have protection equal to or better than murine antibody 13G8 in two highly stringent rodent models of infection. Together, these data expand our understanding regarding this important viral protein and inform the development of broadly effective CCHFV antibody therapeutics.

A novel mucosal bivalent vaccine of EV-A71/EV-D68 adjuvanted with polysaccharides from Ganoderma lucidum protects mice against EV-A71 and EV-D68 lethal challenge

BACKGROUND

Human enteroviruses A71 (EV-A71) and D68 (EV-D68) are the suspected causative agents of hand-foot-and-mouth disease, aseptic meningitis, encephalitis, acute flaccid myelitis, and acute flaccid paralysis in children. Until now, no cure nor mucosal vaccine existed for EV-A71 and EV-D68. Novel mucosal bivalent vaccines are highly important for preventing EV-A71 and EV-D68 infections.

METHODS

In this study, formalin-inactivated EV-A71 and EV-D68 were used as antigens, while PS-G, a polysaccharide from Ganoderma lucidum, was used as an adjuvant. Natural polysaccharides have the characteristics of intrinsic immunomodulation, biocompatibility, low toxicity, and safety. Mice were immunized intranasally with PBS, EV-A71, EV-D68, or EV-A71 + EV-D68, with or without PS-G as an adjuvant.

RESULTS

The EV-A71 + EV-D68 bivalent vaccine generated considerable EV-A71- and EV-D68-specific IgG and IgA titres in the sera, nasal washes, saliva, bronchoalveolar lavage fluid, and feces. These antibodies neutralized EV-D68 and EV-A71 infectivity. They also cross-neutralized infections by different EV-D68 and EV-A71 sub-genotypes. Furthermore, compared with the PBS group, EV-A71 + EV-D68 + PS-G-vaccinated mice exhibited an increased number of EV-D68- and EV-A71-specific IgA- and IgG-producing cells. In addition, T-cell proliferative responses, and IFN-γ and IL-17 secretion in the spleen were substantially induced when PS-G was used as an adjuvant with EV-A71 + EV-D68. Finally, in vivo challenge experiments demonstrated that the immune sera induced by EV-A71 + EV-D68 + PS-G conferred protection in neonate mice against lethal EV-A71 and EV-D68 challenges as indicated by the increased survival rate and decreased clinical score and viral RNA tissue expression. Taken together, all EV-A71/EV-D68 + PS-G-immunized mice developed potent specific humoral, mucosal, and cellular immune responses to EV-D68 and EV-A71 and were protected against them.

CONCLUSIONS

These findings demonstrated that PS-G can be used as a potential adjuvant for EV-A71 and EV-D68 bivalent mucosal vaccines. Our results provide useful information for the further preclinical and clinical development of a mucosal bivalent enterovirus vaccine against both EV-A71 and EV-D68 infections.

Nanodepots Encapsulating a Latency Reversing Agent and Broadly Neutralizing Antibody Enhance Natural Killer Cell Cytotoxicity Against an in vitro Model of Latent HIV

Purpose

Current antiretroviral therapies (ART) for human immunodeficiency virus (HIV) are not curative, as the virus persists in latent reservoirs, requiring lifelong adherence to ART and increasing the risk of co-morbidities. "Shock and kill" approaches to reactivate HIV from latent reservoirs followed by administration of anti-HIV drugs represent a promising strategy for eradicating latent HIV. To achieve effective shock and kill, we describe a strategy to eradicate the HIV reservoir that combines latency reversing agents (LRAs), broadly neutralizing antibodies (bnAbs), and natural killer (NK) cells. This strategy utilizes a polymer nanodepot (ND) that co-encapsulates the LRA and bnAb to reactivate latent infection and elicit enhanced cytotoxicity from co-administered NK cells.

Methods

Poly(lactic-co-glycolic acid) (PLGA) NDs were synthesized using the nanoprecipitation method to co-encapsulate an LRA (TNF-α) and a bnAb (3BNC117) (TNF-α-3BNC117-NDs). ACH-2 cells were used as a cellular model of latent HIV infection. An NK92 subline, genetically modified to constitutively express the Fc receptor CD16, was administered to ACH-2 cells in combination with TNF-α-3BNC117-NDs. ACH-2 cell death and extracellular p24 were measured via flow cytometry and ELISA, respectively.

Results

Stable PLGA NDs co-encapsulated TNF-α and 3BNC117 with high efficiencies and released these agents in physiological conditions. NK92 phenotype remained similar in the presence of TNF-α-3BNC117-NDs. TNF-α released from NDs efficiently reactivated HIV in ACH-2 cells, as measured by a 3.0-fold increase in the frequency of intracellular p24 positive cells. Released 3BNC117 neutralized and bound reactivated virus, targeting 57.5% of total ACH-2 cells. Critically, TNF-α-3BNC117-NDs significantly enhanced NK92 cell-mediated killing of ACH-2 cells (1.9-fold) and reduced extracellular levels of p24 to baseline.

Conclusion

These findings suggest the therapeutic potential of our novel ND-based tripartite strategy to reactivate HIV from latently infected cells, generate an HIV-specific site for bnAb binding, and enhance the killing of reactivated HIV-infected target cells by NK92 cells.

Role of natural killer and B cell interaction in inducing pathogen specific immune responses

The innate lymphoid cell (ILC) system comprising of the circulating and tissue-resident cells is known to clear infectious pathogens, establish immune homeostasis as well as confer antitumor immunity. Human natural killer cells (hNKs) and other ILCs carry out mopping of the infectious pathogens and perform cytolytic activity regulated by the non-adaptive immune system. The NK cells generate immunological memory and rapid recall response tightly regulated by the adaptive immunity. The interaction of NK and B cell, and its role to induce the pathogen specific immunity is not fully understood. Hence, present article sheds light on the interaction between NK and B cells and resulting immune responses in the infectious diseases. The immune responses elicited by the NK-B cell interaction is of particular importance for developing therapeutic vaccines against the infectious pathogens. Further, experimental evidences suggest the immune-response driven by NK cell population elicits the host-specific antibodies and memory B cells. Also, recently developed humanized immune system (HIS) mice and their importance in to understanding the NK-B cell interaction and resulting pathogen specific immunity has been discussed.

Monoclonal CCR5 Antibody: A Promising Therapy for HIV

HIV is one of the world's most devastating viral infections and has claimed tens of millions of lives worldwide since it was first identified in the 1980s. There is no cure for HIV infection. However, with tremendous progress in HIV diagnosis, prevention, and treatment, HIV has become a manageable chronic health disease. CCR5 is an important coreceptor used by HIV to infect target cells, and genetic deficiency of the chemokine receptor CCR5 confers a significant degree of protection against HIV infection. In addition, since CCR5 deficiency does not appear to cause any adverse health effects, targeting this coreceptor is a promising strategy for the treatment and prevention of HIV. Monoclonal antibodies are frequently used as therapeutics for many diseases and therefore are being used as a potential therapy for HIV-1 infection. This review reports on CCR5 antibody research in detail and describes the role and advantages of CCR5 antibodies in HIV prevention or treatment, introduces several main CCR5 antibodies, and discusses the future strategy of antibody-conjugated nanoparticles including the potential challenges. CCR5 antibodies may be a novel therapy for treating HIV infection effectively and could overcome the limitations of the currently available options.

Neutralizing mAbs against SFTS Virus Gn Protein Show Strong Therapeutic Effects in an SFTS Animal Model

Severe fever with thrombocytopenia syndrome (SFTS) is an infectious disease with a high case fatality rate caused by the SFTS virus, and currently there are no approved specific treatments. Neutralizing monoclonal antibodies (mAbs) against the virus could be a therapeutic agent in SFTS treatment, but their development has not sufficiently been carried out. In the present study, mouse and human mAbs exposed to the viral envelope proteins Gn and Gc (16 clones each) were characterized in vitro and in vivo by using recombinant proteins, cell culture with viruses, and an SFTS animal model with IFNAR^-/- mice. Neutralization activities against the recombinant vesicular stomatitis virus bearing SFTS virus Gn/Gc as envelope proteins were observed with three anti-Gn and six anti-Gc mAbs. Therapeutic activities were observed among anti-Gn, but not anti-Gc mAbs with neutralizing activities. These results propose an effective strategy to obtain promising therapeutic mAb candidates for SFTS treatment, and a necessity to reveal precise roles of the SFTS virus Gn/Gc proteins.

microRNA, a Subtle Indicator of Human Cytomegalovirus against Host Immune Cells

Human cytomegalovirus (HCMV) is a double-stranded DNA virus that belongs to the β-herpesvirus family and infects 40–90% of the adult population worldwide. HCMV infection is usually asymptomatic in healthy individuals but causes serious problems in immunocompromised people. We restricted this narrative review (PubMed, January 2022) to demonstrate the interaction and molecular mechanisms between the virus and host immune cells with a focus on HCMV-encoded miRNAs. We found a series of HCMV-encoded miRNAs (e.g., miR-UL112 and miR-UL148D) are explicitly involved in the regulation of viral DNA replication, immune evasion, as well as host cell fate. MiRNA-targeted therapies have been explored for the treatment of atherosclerosis, cardiovascular disease, cancer, diabetes, and hepatitis C virus infection. It is feasible to develop an alternative vaccine to restart peripheral immunity or to inhibit HCMV activity, which may contribute to the antiviral intervention for serious HCMV-related diseases.

Incorporating the Cluster A and V1V2 Targets into a Minimal Structural Unit of the HIV-1 Envelope to Elicit a Cross-Clade Response with Potent Fc-Effector Functions

The generation of a potent vaccine for the prevention and/or control of HIV-1 has been unsuccessful to date, despite decades of research. Existing evidence from both infected individuals and clinical trials support a role for non-neutralizing or weakly neutralizing antibodies with potent Fc-effector functions in the prevention and control of HIV-1 infection. Vaccination strategies that induce such antibodies have proven partially successful in preventing HIV-1 infection. This is largely thought to be due to the polyclonal response that is induced in a vaccine setting, as opposed to the infusion of a single therapeutic antibody, which is capable of diverse Fc-effector functions and targets multiple but highly conserved epitopes. Here, we build on the success of our inner domain antigen, ID2, which incorporates conformational CD4-inducible (CD4i) epitopes of constant region 1 and 2 (C1C2 or Cluster A), in the absence of neutralizing antibody epitopes, into a minimal structural unit of gp120. ID2 has been shown to induce Cluster A-specific antibodies in a BALB/c mouse model with Fc-effector functions against CD4i targets. In order to generate an immunogen that incorporates both epitope targets implicated in the protective Fc-effector functions of antibodies from the only partially successful human vaccine trial, RV144, we incorporated the V1V2 domain into our ID2 antigen generating ID2-V1V2, which we used to immunize in combination with ID2. Immunized BALB/c mice generated both Cluster A- and V1V2-specific antibodies, which synergized to significantly improve the Fc-mediated effector functions compared to mice immunized with ID2 alone. The sera were able to mediate both antibody-dependent cellular cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP). We therefore conclude that ID2-V1V2 + ID2 represents a promising vaccine immunogen candidate for the induction of antibodies with optimal Fc-mediated effector functions against HIV-1.

Complement-Opsonized Nano-Carriers Are Bound by Dendritic Cells (DC) via Complement Receptor (CR)3, and by B Cell Subpopulations via CR-1/2, and Affect the Activation of DC and B-1 Cells

The development of nanocarriers (NC) for biomedical applications has gained large interest due to their potential to co-deliver drugs in a cell-type-targeting manner. However, depending on their surface characteristics, NC accumulate serum factors, termed protein corona, which may affect their cellular binding. We have previously shown that NC coated with carbohydrates to enable biocompatibility triggered the lectin-dependent complement pathway, resulting in enhanced binding to B cells via complement receptor (CR)1/2. Here we show that such NC also engaged all types of splenic leukocytes known to express CR3 at a high rate when NC were pre-incubated with native mouse serum resulting in complement opsonization. By focusing on dendritic cells (DC) as an important antigen-presenting cell type, we show that CR3 was essential for binding/uptake of complement-opsonized NC, whereas CR4, which in mouse is specifically expressed by DC, played no role. Further, a minor B cell subpopulation (B-1), which is important for first-line pathogen responses, and co-expressed CR1/2 and CR3, in general, engaged NC to a much higher extent than normal B cells. Here, we identified CR-1/2 as necessary for binding of complement-opsonized NC, whereas CR3 was dispensable. Interestingly, the binding of complement-opsonized NC to both DC and B-1 cells affected the expression of activation markers. Our findings may have important implications for the design of nano-vaccines against infectious diseases, which codeliver pathogen-specific protein antigen and adjuvant, aimed to induce a broad adaptive cellular and humoral immune response by inducing cytotoxic T lymphocytes that kill infected cells and pathogen-neutralizing antibodies, respectively. Decoration of nano-vaccines either with carbohydrates to trigger complement activation in vivo or with active complement may result in concomitant targeting of DC and B cells and thereby may strongly enhance the extent of dual cellular/humoral immune responses.

Development of antibody-dependent cell cytotoxicity function in HIV-1 antibodies

A prerequisite for the design of an HIV vaccine that elicits protective antibodies is understanding the developmental pathways that result in desirable antibody features. The development of antibodies that mediate antibody-dependent cellular cytotoxicity (ADCC) is particularly relevant because such antibodies have been associated with HIV protection in humans. We reconstructed the developmental pathways of six human HIV-specific ADCC antibodies using longitudinal antibody sequencing data. Most of the inferred naive antibodies did not mediate detectable ADCC. Gain of antigen binding and ADCC function typically required mutations in complementarity determining regions of one or both chains. Enhancement of ADCC potency often required additional mutations in framework regions. Antigen binding affinity and ADCC activity were correlated, but affinity alone was not sufficient to predict ADCC potency. Thus, elicitation of broadly active ADCC antibodies may require mutations that enable high-affinity antigen recognition along with mutations that optimize factors contributing to functional ADCC activity.

Convalescent Plasma Therapy for Management of COVID-19: Perspectives and Deployment in the Current Global Pandemic

The world is striving against the severe crisis of the COVID-19 pandemic. Healthcare professionals are struggling to treat their patients based on nonspecific therapies. Amidst this uncertainty, convalescent plasma therapy (CPT) has appeared to be an interim adjuvant therapy for severely ill patients of COVID-19 until long-term clinical trial treatment options are available. Considering the transfusion-related hazards, especially lung injuries and microbial transmission, where sensitivity is not ensured, rigorous trials should be conducted to determine this therapy's efficacy. Moreover, the ratio of recovered cases to plasma donors is not satisfying, which questioning this therapy's availability and accessibility. Although some countries are making the treatment free, the attributable cost mandates a justification for its suitability and sustainability. Our article aimed to review the published facts and findings of CPT's effectiveness in lowering the mortality rate of COVID-19. This pandemic showed that healthcare systems worldwide need core reform. A unified global collaboration must align and coordinate to face the current pandemic and enhance world readiness for future outbreaks based on health equity and equality.

Effects of gp120 Inner Domain (ID2) Immunogen Doses on Elicitation of Anti-HIV-1 Functional Fc-Effector Response to C1/C2 (Cluster A) Epitopes in Mice

Fc-mediated effector functions of antibodies, including antibody-dependent cytotoxicity (ADCC), have been shown to contribute to vaccine-induced protection from HIV-1 infection, especially those directed against non-neutralizing, CD4 inducible (CD4i) epitopes within the gp120 constant 1 and 2 regions (C1/C2 or Cluster A epitopes). However, recent passive immunization studies have not been able to definitively confirm roles for these antibodies in HIV-1 prevention mostly due to the complications of cross-species Fc–FcR interactions and suboptimal dosing strategies. Here, we use our stabilized gp120 Inner domain (ID2) immunogen that displays the Cluster A epitopes within a minimal structural unit of HIV-1 Env to investigate an immunization protocol that induces a fine-tuned antibody repertoire capable of an effective Fc-effector response. This includes the generation of isotypes and the enhanced antibody specificity known to be vital for maximal Fc-effector activities, while minimizing the induction of isotypes know to be detrimental for these functions. Although our studies were done in in BALB/c mice we conclude that when optimally titrated for the species of interest, ID2 with GLA-SE adjuvant will elicit high titers of antibodies targeting the Cluster A region with potent Fc-mediated effector functions, making it a valuable immunogen candidate for testing an exclusive role of non-neutralizing antibody response in HIV-1 protection in vaccine settings.

Defining rules governing recognition and Fc-mediated effector functions to the HIV-1 co-receptor binding site

BACKGROUND

The binding of HIV-1 Envelope glycoproteins (Env) to host receptor CD4 exposes vulnerable conserved epitopes within the co-receptor binding site (CoRBS) which are required for the engagement of either CCR5 or CXCR4 co-receptor to allow HIV-1 entry. Antibodies against this region have been implicated in the protection against HIV acquisition in non-human primate (NHP) challenge studies and found to act synergistically with antibodies of other specificities to deliver effective Fc-mediated effector function against HIV-1-infected cells. Here, we describe the structure and function of N12-i2, an antibody isolated from an HIV-1-infected individual, and show how the unique structural features of this antibody allow for its effective Env recognition and Fc-mediated effector function.

RESULTS

N12-i2 binds within the CoRBS utilizing two adjacent sulfo-tyrosines (TYS) for binding, one of which binds to a previously unknown TYS binding pocket formed by gp120 residues of high sequence conservation among HIV-1 strains. Structural alignment with gp120 in complex with the co-receptor CCR5 indicates that the new pocket corresponds to TYS at position 15 of CCR5. In addition, structure-function analysis of N12-i2 and other CoRBS-specific antibodies indicates a link between modes of antibody binding within the CoRBS and Fc-mediated effector activities. The efficiency of antibody-dependent cellular cytotoxicity (ADCC) correlated with both the level of antibody binding and the mode of antibody attachment to the epitope region, specifically with the way the Fc region was oriented relative to the target cell surface. Antibodies with poor Fc access mediated the poorest ADCC whereas those with their Fc region readily accessible for interaction with effector cells mediated the most potent ADCC.

CONCLUSION

Our data identify a previously unknown binding site for TYS within the assembled CoRBS of the HIV-1 virus. In addition, our combined structural-modeling-functional analyses provide new insights into mechanisms of Fc-effector function of antibodies against HIV-1, in particular, how antibody binding to Env antigen affects the efficiency of ADCC response.

HIV/AIDS Research for the Future

Despite significant progress, several questions related to HIV infection remain to be addressed. Here, I provide my perspective on four key areas that need further research to inform curative and preventive measures against HIV/AIDS.

Flow Cytometry Analysis of HIV-1 Env Conformations at the Surface of Infected Cells and Virions: Role of Nef, CD4, and SERINC5

The HIV-1 Env protein is exposed at the surface of virions and infected cells. Env fluctuates between different closed and open structural states and these conformations influence both viral infectivity and sensitivity to antibody binding and neutralization. We established a flow virometry assay to visualize Env proteins at the surface of human immunodeficiency virus type 1 (HIV-1) virions. The assay is performed on ultracentrifuged fluorescent viral particles that are stained with a panel of broadly neutralizing antibodies (bNAbs) and nonneutralizing antibodies (nnAbs) that probe different epitopes of Env. We used this assay to compare Env at the surface of producer cells and viral particles and to analyze the effect of Nef, CD4, and SERINC5 on Env accessibility to antibodies. We studied the laboratory-adapted strain NL4-3 and two transmitted/founder viruses, THRO and CH058. We confirm that antibody accessibility varies between viral strains and show that Nef, CD4, and SERINC5 additively impact Env conformations. We further demonstrate that the Env accessibility profile on virions is globally similar to that observed on HIV-1-infected cells, with some noticeable differences. For instance, nnAbs bind to virions more efficiently than to producer cells, likely reflecting changes in Env conformational states on mature viral particles. This test complements other techniques and provides a convenient and simple tool for quantifying and probing the structure of Env at the virion surface and to analyze the impact of viral and cellular proteins on these parameters.IMPORTANCE HIV-1 Env conformation is one of the key parameters determining viral infectivity. The flow virometry-based assay developed in this study allows for the characterization of proteins incorporated in HIV-1 particles. We studied the conformation of HIV-1 Env and the impact that the viral protein Nef and the cellular proteins CD4 and SERINC5 have on Env accessibility to antibodies. Our assay permitted us to highlight some noticeable differences in the conformation of Env between producer cells and viral particles. It contributes to a better understanding of the actual composition of HIV-1 particles.

A review of CCR5 antibodies against HIV: current and future aspects

HIV is one of the most devastating viral infections the world has ever encountered. Ever since HIV was first identified in the 1980s, it has claimed millions of lives worldwide. There has been tremendous research and development in the diagnosis, prevention and treatment of HIV. Small molecules have been shown to reduce the virus to nondetectable level in human plasma, however, there are reservoirs of latent virus that reemerge if antiretroviral therapy is stopped. There is no vaccine to prevent or cure HIV. A significant amount of research has been reported in the literature regarding antibodies for CCR5, a HIV entry host receptor. This report describes the role of CCR5 antibody in HIV prevention/treatment and how antibody-conjugated nanoparticles could be a future strategy with the potential to effectively eradicate the virus from the human system.

Update on Fc-Mediated Antibody Functions Against HIV-1 Beyond Neutralization

Antibodies (Abs) are the major component of the humoral immune response and a key player in vaccination. The precise Ab-mediated inhibitory mechanisms leading to in vivo protection against HIV have not been elucidated. In addition to the desired viral capture and neutralizing Ab functions, complex Ab-dependent mechanisms that involve engaging immune effector cells to clear infected host cells, immune complexes, and opsonized virus have been proposed as being relevant. These inhibitory mechanisms involve Fc-mediated effector functions leading to Ab-dependent cellular cytotoxicity, phagocytosis, cell-mediated virus inhibition, aggregation, and complement inhibition. Indeed, the decreased risk of infection observed in the RV144 HIV-1 vaccine trial was correlated with the production of non-neutralizing inhibitory Abs, highlighting the role of Ab inhibitory functions besides neutralization. Moreover, Ab isotypes and subclasses recognizing specific HIV envelope epitopes as well as pecular Fc-receptor polymorphisms have been associated with disease progression. These findings further support the need to define which Fc-mediated Ab inhibitory functions leading to protection are critical for HIV vaccine design. Herein, based on our previous review Su & Moog Front Immunol 2014, we update the different inhibitory properties of HIV-specific Abs that may potentially contribute to HIV protection.

Natural Killer Cell Regulation of B Cell Responses in the Context of Viral Infection

Secretion of both neutralizing and nonneutralizing virus-specific antibodies by B cells is a key component of immune control of many virus infections and a critical benchmark of successful preventative vaccines. Natural killer (NK) cells also play a vital role in antiviral immune defense via cytolytic elimination of infected cells and production of proinflammatory antiviral cytokines. Accumulating evidence points to multifaceted crosstalk between NK cells and antiviral B cell responses that can determine virus elimination, pathogenesis of infection, and efficacy of vaccine-elicited protection. These outcomes are a result of both positive and negative influences of NK cells on the B cell responses, as well as canonical antiviral killing of infected B cells. On one hand, NK cell-derived cytokines such as interferon-gamma (IFN-γ) may promote B cell activation and enhance immunoglobulin production. In contrast, NK cell immunoregulatory killing of CD4 T cells can limit affinity maturation in germinal centers resulting in weak infection or vaccine induction of antiviral neutralizing antibodies. In this review, we will discuss these and other dueling contributions of NK cells to B cell responses during virus infection or vaccination.

Anti-Tat Immunity in HIV-1 Infection: Effects of Naturally Occurring and Vaccine-Induced Antibodies Against Tat on the Course of the Disease

HIV-1 Tat is an essential protein in the virus life cycle, which is required for virus gene expression and replication. Most Tat that is produced during infection is released extracellularly and it plays a key role in HIV pathogenesis, including residual disease upon combination antiretroviral therapy (cART). Here, we review epidemiological and experimental evidence showing that antibodies against HIV-1 Tat, infrequently occurring in natural infection, play a protective role against disease progression, and that vaccine targeting Tat can intensify cART. In fact, Tat vaccination of subjects on suppressive cART in Italy and South Africa promoted immune restoration, including CD4+ T-cell increase in low immunological responders, and a reduction of proviral DNA even after six years of cART, when both CD4+ T-cell gain and DNA decay have reached a plateau. Of note, DNA decay was predicted by the neutralization of Tat-mediated entry of Env into dendritic cells by anti-Tat antibodies, which were cross-clade binding and neutralizing. Anti-Tat cellular immunity also contributed to the DNA decay. Based on these data, we propose the Tat therapeutic vaccine as a pathogenesis-driven intervention that effectively intensifies cART and it may lead to a functional cure, providing new perspectives and opportunities also for prevention and virus eradication strategies.

Induction of Fc-Mediated Effector Functions Against a Stabilized Inner Domain of HIV-1 gp120 Designed to Selectively Harbor the A32 Epitope Region

Recent clinical trials and studies using nonhuman primates (NHPs) suggest that antibody-mediated protection against HIV-1 will require α-HIV envelope humoral immunity beyond direct neutralization to include Fc-receptor (FcR) mediated effector functions such as antibody-dependent cellular cytotoxicity (ADCC). There is also strong evidence indicating that the most potent ADCC response in humans is directed toward transitional non-neutralizing epitopes associated with the gp41-interactive face of gp120, particularly those within the first and second constant (C1–C2) region (A32-like epitopes). These epitopes were shown to be major targets of ADCC responses during natural infection and have been implicated in vaccine-induced protective immunity. Here we describe the immunogenicity of ID2, an immunogen consisting of the inner domain of the clade A/E 93TH057 HIV-1 gp120 expressed independently of the outer domain (OD) and stabilized in the CD4-bound conformation to harbor conformational A32 region epitopes within a minimal structural unit of HIV-1 Env. ID2 induced A32-specific antibody responses in BALB/c mice when injected alone or in the presence of the adjuvants Alum or GLA-SE. Low α-ID2 titers were detected in mice immunized with ID2 alone whereas robust responses were observed with ID2 plus adjuvant, with the greatest ID2 and A32-specific titers observed in the GLA-SE group. Only sera from groups immunized in the presence of GLA-SE were capable of mediating significant ADCC using NKr cells sensitized with recombinant BaL gp120 as targets and human PBMCs as effectors. A neutralization response to a tier 2 virus was not observed. Altogether, our studies demonstrate that ID2 is highly immunogenic and elicits A32-specific ADCC responses in an animal host. The ID2 immunogen has significant translational value as it can be used in challenge studies to evaluate the role of non-neutralizing antibodies directed at the A32 subregion in HIV-1 protection.

From Rhesus macaque to human: structural evolutionary pathways for immunoglobulin G subclasses

The Old World monkey, Rhesus macaque (Macaca mulatta, Mm), is frequently used as a primate model organism in the study of human disease and to test new vaccines/antibody treatments despite diverging before chimpanzees and orangutans. Mm and humans share 93% genome identity with substantial differences in the genes of the adaptive immune system that lead to different functional IgG subclass characteristics, Fcγ receptors expressed on innate immune cells, and biological interactions. These differences put limitations on Mm use as a primary animal model in the study of human disease and to test new vaccines/antibody treatments. Here, we comprehensively analyzed molecular properties of the Fc domain of the four IgG subclasses of Rhesus macaque to describe potential mechanisms for their interactions with effector cell Fc receptors. Our studies revealed less diversity in the overall structure among the Mm IgG Fc, with MmIgG1 Fc being the most structurally like human IgG3, although its CH2 loops and N297 glycan mobility are comparable to human IgG1. Furthermore, the Fcs of Mm IgG3 and 4 lack the structural properties typical for their human orthologues that determine IgG3's reduced interaction with the neonatal receptor and IgG4's ability for Fab-arm exchange and its weaker Fcγ receptor interactions. Taken together, our data indicate that MmIgG1-4 are less structurally divergent than the human IgGs, with only MmIgG1 matching the molecular properties of human IgG1 and 3, the most active IgGs in terms of Fcγ receptor binding and Fc-mediated functions. PDB accession numbers for deposited structures are 6D4E, 6D4I, 6D4M, and 6D4N for MmIgG1 Fc, MmIgG2 Fc, MmIgG3 Fc, and MmIgG4 Fc, respectively.

Antibody-mediated protection against Ebola virus

Recent Ebola virus disease epidemics have highlighted the need for effective vaccines and therapeutics to prevent future outbreaks. Antibodies are clearly critical for control of this deadly disease; however, the specific mechanisms of action of protective antibodies have yet to be defined. In this Perspective we discuss the antibody features that correlate with in vivo protection during infection with Ebola virus, based on the results of a systematic and comprehensive study of antibodies directed against this virus. Although neutralization activity mediated by the Fab domains of the antibody is strongly correlated with protection, recruitment of immune effector functions by the Fc domain has also emerged as a complementary, and sometimes alternative, route to protection. For a subset of antibodies, Fc-mediated clearance and killing of infected cells seems to be the main driver of protection after exposure and mirrors observations in vaccination studies. Continued analysis of antibodies that achieve protection partially or wholly through Fc-mediated functions, the precise functions required, the intersection with specificity and the importance of these functions in different animal models is needed to identify and begin to capitalize on Fc-mediated protection in vaccines and therapeutics alike.

SOSIP Changes Affect Human Immunodeficiency Virus Type 1 Envelope Glycoprotein Conformation and CD4 Engagement

The entry of human immunodeficiency virus into host cells is mediated by the envelope glycoprotein (Env) trimeric spike, which consists of three exterior gp120 subunits and three transmembrane gp41 subunits. The trimeric Env undergoes extensive conformational rearrangement upon interaction with the CD4 receptor, transitioning from the unliganded, "closed" State 1 to more-open downstream State 2 and State 3 conformations. Changes in "restraining" amino acid residues, such as leucine 193 and isoleucine 423, destabilize State 1 Env, which then assumes entry-competent, downstream conformations. The introduction of an artificial disulfide bond linking the gp120 and gp41 subunits (SOS) in combination with the I559P (IP) change has allowed structural characterization of soluble gp140 (sgp140) trimers. The conformation of these SOSIP-stabilized sgp140 trimers has been suggested to represent the closed native State 1 conformation. Here we compare the impact on the membrane Env conformation of the SOSIP changes with that of the well-characterized changes (L193R and I423A) that shift Env to downstream States 2 and 3. The results presented here suggest that the SOSIP changes stabilize Env in a conformation that differs from State 1 but also from the downstream Env conformations stabilized by L193R or I423A.IMPORTANCE The human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) trimer is triggered by receptor binding to mediate the entry of the virus into cells. Most structural studies of Env trimers have utilized truncated soluble gp140 Envs stabilized with the I559P and SOS changes. Here we present evidence indicating that these stabilizing changes have a profound impact on the conformation of Env, moving Env away from the native pretriggered Env conformation. Our studies underscore the need to acquire structural information on the pretriggered Env conformation, which is recognized by most broadly reactive neutralizing antibodies.

Systematic Analysis of Monoclonal Antibodies against Ebola Virus GP Defines Features that Contribute to Protection

Antibodies are promising post-exposure therapies against emerging viruses, but which antibody features and in vitro assays best forecast protection are unclear. Our international consortium systematically evaluated antibodies against Ebola virus (EBOV) using multidisciplinary assays. For each antibody, we evaluated epitopes recognized on the viral surface glycoprotein (GP) and secreted glycoprotein (sGP), readouts of multiple neutralization assays, fraction of virions left un-neutralized, glycan structures, phagocytic and natural killer cell functions elicited, and in vivo protection in a mouse challenge model. Neutralization and induction of multiple immune effector functions (IEFs) correlated most strongly with protection. Neutralization predominantly occurred via epitopes maintained on endosomally cleaved GP, whereas maximal IEF mapped to epitopes farthest from the viral membrane. Unexpectedly, sGP cross-reactivity did not significantly influence in vivo protection. This comprehensive dataset provides a rubric to evaluate novel antibodies and vaccine responses and a roadmap for therapeutic development for EBOV and related viruses.

Incomplete Downregulation of CD4 Expression Affects HIV-1 Env Conformation and Antibody-Dependent Cellular Cytotoxicity Responses

HIV-1-infected cells expressing envelope glycoproteins (Env) in the CD4-bound conformation on their surfaces are targeted by antibody-dependent cellular cytotoxicity (ADCC) mediated by CD4-induced (CD4i) antibodies and sera from HIV-1-infected individuals (HIV⁺ sera). By downregulating the surface expression of CD4, Nef prevents Env-CD4 interaction, thus protecting HIV-1-infected cells from ADCC. HIV-1 infectious molecular clones (IMCs) are widely used to measure ADCC. In order to facilitate the identification of infected cells and high-throughput ADCC analysis, reporter genes (e.g., the Renilla luciferase [LucR] gene) are often introduced into IMC constructs. We evaluated the susceptibility of HIV-1-infected CD4⁺ T lymphocytes to ADCC using a panel of parental IMCs and derivatives that expressed the LucR reporter gene, utilizing different molecular strategies, including one specifically designed to retain Nef expression. We found that in some of these constructs, Nef expression in CD4⁺ T cells was suboptimal, and consequently, CD4 downregulation was incomplete. CD4 molecules remaining on the cell surface resulted in the exposure of ADCC-mediating CD4i epitopes on Env and a dramatic increase in the susceptibility of the infected cells to ADCC. Strikingly, protection from ADCC was observed when cells were infected with the parental IMC, which exhibited strong CD4 downregulation. This discrepancy between the parental and Nef-impaired viruses was independent of the strains of Env expressed, but rather, it was correlated with the levels of CD4 surface expression. Overall, our results indicate that caution should be taken when selecting IMCs for ADCC measurements and that CD4 downregulation needs to be carefully monitored when drawing conclusions about the nature and magnitude of ADCC.

IMPORTANCE In-depth understanding of the susceptibility of HIV-1-infected cells to ADCC might help establish correlates of vaccine protection and guide the development of HIV-1 vaccine strategies. Different ADCC assays have been developed, including those using infectious molecular clones (IMCs) carrying a LucR reporter gene that greatly facilitates large-scale quantitative analysis. We previously reported different molecular strategies for introducing LucR while maintaining Nef expression and function and, consequently, CD4 surface downregulation. Here, we demonstrate that utilizing IMCs that exhibit impaired Nef expression can have undesirable consequences due to incomplete CD4 downregulation. CD4 molecules remaining on the cell surface resulted in the exposure of ADCC-mediating CD4i epitopes on Env and a dramatic increase in the susceptibility of the infected cells to ADCC. Overall, our results indicate that CD4 downregulation needs to be carefully monitored when drawing conclusions about the nature and magnitude of ADCC.

Passive and active antibody studies in primates to inform HIV vaccines

INTRODUCTION

Prevention of infection remains the ultimate goal for HIV vaccination, and there is compelling evidence that antibodies directed to Envelope are necessary to block infection. Generating antibodies that are sufficiently broad, potent, and sustained to block infection by the diverse HIV-1 strains circulating worldwide remains an area of intense study.

AREAS COVERED

In this review, we have summarized progress from publications listed as PubMed citations in 2016-17 in the areas of passive antibody studies using human neutralizing monoclonal antibodies in nonhuman primates, HIV Envelope vaccine development and active vaccination studies to generate potent neutralizing antibodies.

EXPERT COMMENTARY

Passive transfer studies in nonhuman primates using human neutralizing monoclonal antibodies have informed the potency, specificity, and cooperativity of antibodies needed to prevent infection, leading to clinical studies now testing potent antibodies for prevention of HIV. Progress in understanding the structure of Envelope has led to novel vaccine constructs, including mimetics, scaffolds and native-like proteins. As yet, no single approach ensures protection against the circulating global HIV-1 strains, but there is progress in understanding why, and intense research continues in these and other areas for a solution. We offer perspectives on how this knowledge may shape the design of future HIV vaccines.

Envelope glycoproteins sampling states 2/3 are susceptible to ADCC by sera from HIV-1-infected individuals

Recent analysis of HIV-1 envelope glycoproteins (Env) dynamics showed that the unliganded Env trimer can potentially sample three conformations: a metastable "closed" conformation (State 1), an "open" CD4-bound conformation (State 3), and an intermediate "partially open" conformation (State 2). HIV-1 evolved several mechanisms to avoid "opening" its Env in order to evade immune responses such as antibody-dependent cellular cytotoxicity (ADCC), which preferentially targets Envs in the CD4-bound conformation on the surface of infected cells. Here we took advantage of a well-characterized single-residue change in the gp120 trimer association domain to modify Env conformation and evaluate its impact on ADCC responses. We found that cells infected with viruses expressing Env stabilized in States 2/3 become highly susceptible to ADCC responses by sera from HIV-1-infected individuals. Our results indicate that the conformations spontaneously sampled by the Env trimer at the surface of infected cells has a significant impact on ADCC responses.

Non-Neutralizing Antibodies Directed against HIV and Their Functions

B cells produce a plethora of anti-HIV antibodies (Abs) but only few of them exhibit neutralizing activity. This was long considered a profound limitation for the enforcement of humoral immune responses against HIV-1 infection, especially since these neutralizing Abs (nAbs) are extremely difficult to induce. However, increasing evidence shows that additional non-neutralizing Abs play a significant role in decreasing the viral load, leading to partial and sometimes even total protection. Mechanisms suspected to participate in protection are numerous. They involve the Fc domain of Abs as well as their Fab part, and consequently the induced Ab isotype will be determinant for their functions, as well as the quantity and quality of the Fc-receptors (FcRs) expressed on immune cells. Fc-mediated inhibitory functions, such as Ab-dependent cellular cytotoxicity, antibody-dependent cellular phagocytosis, aggregation, and even immune activation have been proposed. However, as for nAbs, the non-neutralizing activities are limited to a subset of anti-HIV Abs. An improved in-depth characterization of the Abs displaying these functional responses is required for the development of new vaccination strategies, which aim to selectively trigger the B cells able to induce the right functional Ab combinations both at the right place and at the right time. This review summarizes our current knowledge on non-neutralizing functional inhibitory Abs and discusses the potential benefit of inducing them via vaccination. We also provide new insight into the roles of the FcγR-mediated Ab therapeutics in clinical trials for HIV diseases.

Impact of HIV-1 Envelope Conformation on ADCC Responses

HIV-1 envelope glycoproteins (Env) represent the only virus-specific antigen exposed at the surface of infected cells. In its unliganded form, Env from primary viruses samples a 'closed' conformation (State 1), which is preferentially recognized by broadly neutralizing antibodies (bNAbs). CD4 engagement drives Env into an intermediate 'partially open' (State 2) and then into the 'open' CD4-bound conformation (State 3). Emerging evidence suggests a link between Env conformation and Ab-dependent cellular cytotoxicity (ADCC). HIV-1-infected cells exposing Env in the CD4-bound conformation are susceptible to ADCC mediated by CD4-induced Abs and HIV+sera. Cells exposing State 1 Env are susceptible to ADCC mediated by bNAbs. Here, we discuss how Env conformation affects ADCC responses and in vitro measurements.

Targeting the Late Stage of HIV-1 Entry for Antibody-Dependent Cellular Cytotoxicity: Structural Basis for Env Epitopes in the C11 Region

Antibodies can have an impact on HIV-1 infection in multiple ways, including antibody-dependent cellular cytotoxicity (ADCC), a correlate of protection observed in the RV144 vaccine trial. One of the most potent ADCC-inducing epitopes on HIV-1 Env is recognized by the C11 antibody. Here, we present the crystal structure, at 2.9 Å resolution, of the C11-like antibody N12-i3, in a quaternary complex with the HIV-1 gp120, a CD4-mimicking peptide M48U1, and an A32-like antibody, N5-i5. Antibody N12-i3 recognizes an epitope centered on the N-terminal "eighth strand" of a critical β sandwich, which our analysis indicates to be emblematic of a late-entry state, after the gp120 detachment. In prior entry states, this sandwich comprises only seven strands, with the eighth strand instead pairing with a portion of the gp120 C terminus. The conformational gymnastics of HIV-1 gp120 thus includes altered β-strand pairing, possibly to reduce immunogenicity, although nevertheless still recognized by the human immune system.

BST-2 Expression Modulates Small CD4-Mimetic Sensitization of HIV-1-Infected Cells to Antibody-Dependent Cellular Cytotoxicity

Antibodies recognizing conserved CD4-induced (CD4i) epitopes on human immunodeficiency virus type 1 (HIV-1) Env and able to mediate antibody-dependent cellular cytotoxicity (ADCC) have been shown to be present in sera from most HIV-1-infected individuals. These antibodies preferentially recognize Env in its CD4-bound conformation. CD4 downregulation by Nef and Vpu dramatically reduces exposure of CD4i HIV-1 Env epitopes and therefore reduce the susceptibility of HIV-1-infected cells to ADCC mediated by HIV-positive (HIV+) sera. Importantly, this mechanism of immune evasion can be circumvented with small-molecule CD4 mimetics (CD4mc) that are able to transition Env into the CD4-bound conformation and sensitize HIV-1-infected cells to ADCC mediated by HIV+ sera. However, HIV-1 developed additional mechanisms to avoid ADCC, including Vpu-mediated BST-2 antagonism, which decreases the overall amount of Env present at the cell surface. Accordingly, BST-2 upregulation in response to alpha interferon (IFN-α) was shown to increase the susceptibility of HIV-1-infected cells to ADCC despite the activity of Vpu. Here we show that BST-2 upregulation by IFN-β and interleukin-27 (IL-27) also increases the surface expression of Env and thus boosts the ability of CD4mc to sensitize HIV-1-infected cells to ADCC by sera from HIV-1-infected individuals.IMPORTANCE HIV-1 evolved sophisticated strategies to conceal Env epitopes from ADCC-mediating antibodies present in HIV+ sera. Vpu-mediated BST-2 downregulation was shown to decrease ADCC responses by limiting the amount of Env present at the cell surface. This effect of Vpu was shown to be attenuated by IFN-α treatment. Here we show that in addition to IFN-α, IFN-β and IL-27 also affect Vpu-mediated BST-2 downregulation and greatly enhance ADCC responses against HIV-1-infected cells in the presence of CD4mc. These findings may inform strategies aimed at HIV prevention and eradication.